Myotonic dystrophy (DM) results from the amplification of an unstable (CTG)n sequence in the 3' untranslated region of the myotonin-protein kinase (MT-PK) gene. The expression of the enlarged allele in DM patients with a number of repeats below or beyond 200, was analysed by three different groups. Two groups showed a decreased or absent expression of mutant alleles in DM adults, in congenitally affected infants (CDM) and in an affected fetus. On the contrary, another group reported the increased expression of the mutated allele in several tissues of a CDM infant. These discrepancies may be explained by the different methods used, the small number of patients, the individuals and tissues used as controls, or reflect the use of primers located in different regions of the MT-PK gene.